Digital terrestrial broadcasting, and IEEE 802.11a which is a transmission standard of the wireless LAN, employ an OFDM system. The OFDM system is a multicarrier system in which multiple subcarriers are multiplexed and transmitted within a bandwidth of one channel. The OFDM system is known as a system which is highly resistant to multipath interference since the symbol interval length is long in comparison with a case of a single carrier transmission. In addition, by providing so-called a guard interval which is obtained by cyclically copying a part of an effective symbol, an advantage that multipath does not cause intersymbol interference is obtained as long as the multipath occurs within the guard interval.
In Japan and Europe, transmission standards used for digital terrestrial broadcasting are called an ISDB-T system and a DVB-T system, respectively. Hereinafter, transmission processing and reception processing which are common to the ISDB-T system and the DVB-T system will be described.
In the ISDB-T system and the DVB-T system, pilot signals whose amplitude and phase are already known are scattered with respect to the frequency domain and inserted in subcarriers. These pilot signals are referred to as scattered pilot signals (hereinafter, marked down as SP signals). FIG. 24 shows an arrangement of the SP signals. In FIG. 24, the SP signals are located, in the frequency (subcarrier) direction and time (symbol) direction, at carrier positions where a symbol number n and a carrier number k satisfy k=3(n mod 4)+12p (mod indicates remainder operation, and p is an integer number). That is, the SP signal is iteratively located in cycles of four symbols and the SP signals are shifted by three carriers with respect to each symbol. The SP signals which have been located in such a manner are modulated into binary data based on particular patterns determined depending on the carrier positions of the SP signals, and then transmitted.
Moreover, in the ISDB-T system and the DVB-T system, by using carriers at which the SP signals are not located, information transmission signals are modulated by a method such as QPSK, 16 QAM, 64 QAM, or the like, and then transmitted.
FIG. 25 shows a configuration of a conventional OFDM transmitting apparatus 1000 in which the ISDB-T system and the DVB-T system are used. The conventional OFDM transmitting apparatus 1000 includes an error correction coding section 1001, a mapping section 1002, an interleaving section 1003, a frame constructing section 1004, an IFFT processing section 1005, a guard interval adding section 1006, an RF frequency conversion section 1007, and an antenna 1008.
Hereinafter, operation of the conventional OFDM transmitting apparatus 1000 will be described.
The error correction coding section 1001 performs error correction coding processing on an information transmission signal. The mapping section 1002 maps data obtained through the error correction coding processing, through QPSK, 16 QAM, 64 QAM, or the like. The interleaving section 1003 performs interleaving, such as time interleaving, frequency interleaving, or the like, on data obtained by the mapping, on a subcarrier symbol basis. The frame constructing section 1004 arranges each carrier symbol unit of the interleaved data with SP signals in accordance with the arrangement diagram shown in FIG. 24, thereby constructing frames. The IFFT processing section 1005 transforms the constructed frames of data into a signal in the time domain. The guard interval adding section 1006 adds guard intervals to the data transformed into a signal in the time domain. As shown in FIG. 26, the guard intervals are obtained by cyclically copying and adding the rear part of the effective symbol to a front part of the symbol. That is, one symbol interval includes a guard interval and the effective symbol interval which follows the guard interval. The RF frequency conversion section 1007 converts, to an RF frequency, the signal to which the guard intervals have been added. The antenna 1008 transmits the signal converted to an RF frequency.
FIG. 27 shows a configuration of a conventional OFDM receiving apparatus 1100 in which the ISDB-T system and the DVB-T system are used. The conventional OFDM receiving apparatus 1100 includes an antenna 1101, a tuning section 1102, a demodulation section 1111, and an error correction decoding section 1108. Moreover, the demodulation section 1111 includes an A/D conversion section 1103, a quadrature detection section 1104, a synchronization section 1105, an FFT processing section 1106, and an equalization section 1107.
Hereinafter, operation of the conventional OFDM transmitting apparatus 1100 will be described.
The antenna 1101 receives a radio wave, and the tuning section 1102 selectively receives an OFDM signal of a desired channel and downconverts the OFDM signal to a selected frequency. The A/D conversion section 1103 A/D-converts the downconverted OFDM signal. The quadrature detection section 1104 performs quadrature detection of the A/D-converted digital signal. The synchronization section 1105 performs synchronization processing such as symbol synchronization, sampling frequency synchronization, frequency synchronization, and the like, and determines an FFT window position. The FFT processing section 1106 transforms a signal in the time domain into a signal in the frequency domain through FFT processing. The equalization section 1107 calculates a transmission path frequency response from the signal in the frequency domain outputted by the FFT processing section 1106, and then performs equalization processing on the signal in the frequency domain based on the transmission path frequency response. The error correction decoding section 1108 performs error correction processing on the signal on which equalization processing has been performed, and thereby outputs a TS (Transport Stream) signal.
FIG. 28 shows in detail a configuration of the equalization section 1107 in FIG. 27. The equalization section 1107 includes an SP demodulation section 1201, a symbol interpolation section 1202, a carrier interpolation section 1203, a delay section 1204, and a complex division section 1205.
Hereinafter, operation of the equalization section 1107 will be described.
The SP demodulation section 1201 extracts the SP signals from the signal in the frequency domain outputted by the FFT processing section 1106, in accordance with the arrangement of the SP signals shown in FIG. 24. The SP demodulation section 1201 performs complex division on the SP signals by predetermined patterns which are determined depending on the carrier positions of the SP signals, and thereby outputs transmission path characteristic estimates for the SP signal positions. The symbol interpolation section 1202 interpolates the transmission path characteristics for the SP signal positions in the time-axis direction as shown in FIG. 29, and thereby outputs transmission path characteristic estimates for each three carriers. The carrier interpolation section 1203 interpolates the transmission path characteristics for each three carriers in the frequency-axis direction as shown in FIG. 30, and thereby outputs transmission path characteristic estimates for all carrier positions. In the above-described transmission path estimation method, first, interpolation is performed in the time-axis direction by using the SP signal at four-symbol intervals. Hereinafter, this method is referred to as “4-symbol equalization”.
The delay section 1204 performs delay matching between the transmission path characteristic estimates for all carrier positions outputted by the carrier interpolation section 1203, and the signal in the frequency domain outputted by the FFT processing section 1106. The complex division section 1205 complex-divides the signal in the frequency domain outputted by the delay section 1204 by the transmission path characteristic estimates for all the carrier positions outputted by the carrier interpolation section 1203, and whereby the received signal is equalized.
On the other hand, FIG. 31 shows a configuration of an equalization section 1300 disclosed in a Non-Patent Document. The equalization section 1300 in FIG. 31 is obtained by removing the symbol interpolation section 1202 from the equalization section 1107 in FIG. 28 and including a carrier interpolation section 1301 instead of the carrier interpolation section 1203. As shown in FIG. 32, the equalization section 1300 causes the carrier interpolation section 1301 to interpolate, in the frequency-axis direction, the transmission path characteristics for the SP signal positions outputted by the SP demodulation section 1201 without interpolating the transmission path characteristics in the time-axis direction, and thereby outputs the transmission path characteristic estimates for all the carrier positions. That is, in this transmission estimation method, calculation is independently performed for each single symbol to obtain the transmission path characteristic estimates for all the carrier positions. Hereinafter, this method is referred to as “1-symbol equalization”.
When using the 1-symbol equalization, resolution in the frequency direction is lowered in comparison with when using the 4-symbol equalization, and therefore estimation accuracy in the frequency direction is deteriorated. However, in high-speed mobile reception, estimation accuracy in the time direction can be enhanced.
FIG. 33 shows a configuration of an equalization section 1400 disclosed in Patent Document 1. The equalization section 1400 in FIG. 33 is obtained by adding the carrier interpolation section 1301 for the 1-symbol equalization shown in FIG. 31, an amplitude variation detecting section 1401, and a switching section 1402 to the equalization section 1107 in FIG. 28. In the equalization section 1400, the amplitude variation detecting section 1401 detects the amplitude variation rate of the carrier, and switches between the 1-symbol equalization and the 4-symbol equalization in accordance with the result of the detection.
With this configuration, when the amplitude variation rate is large, the 1-symbol equalization is performed and thereby estimation accuracy in the time direction can be enhanced in high-speed mobile reception, and when the amplitude variation rate is small, the 4-symbol equalization is performed and thereby estimation accuracy in the frequency characteristic can be maintained.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-140987    Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-336279    Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-312027    Non-Patent Document 1: Kimura and others “Performance of high-speed mobile reception for digital terrestrial broadcasting by one symbol channel estimation”, The Technical Report on the Institute of Image Information and Television Engineers, BCT 2005-69, Jun. 2005.